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  • Selec-Trac 4-wheel drive = 2wd mode disengages in (drive) forward motion

    I have not been here in a while but I took a lot of this information and work on my 1983 Jeep Laredo AMC 360.

    I remove the smog pump and most of the factory vacuum lines (which all seem to be cracked) found almost all vacuum from the factory carb (2 barrel) was in the same condition as the smog components.

    Well New intake manifold gaskets, the carb insulator & gaskets, plugged up the extra vacuum ports - and I found that the factory 2bbl carb rebuild is leaking vacuum at the throttle shaft! I mean it is whistling at idle!!

    Also since I cut down 80% of the vacuum leaks the Vacuum switch for the Selec-Trac (which was not working) began to operate!! It worked in 2wd, 4HI & 4LO for almost two months (since November snow fall here in Central Illinois).

    WELL NOW this is what is happening...I have gone thru some deep snow / slush & water today. Nothing bad but I had to switch into the 4HI. I shifted the Auto-trans into Neutral (the vehicle may have been rolling backwards for a foot or two) switch the Selec-Trac to 4HI put the tranny into Drive & drove out of the underpass on to level ground (less than 1/8 mile) I stop at a intersection, so I put the tranny into N and switch the Selec-Trac to 2wd mode & put the tranny into Drive and continued drive around 35 - 45 mph

    I felt a huge metal bang / transfer case noise (chain) and I loss power to the rear wheels. I tried to putting the tranny in Reverse (still in 2WD mode) and I could hear the front axle bang but I began moving backward with no noise or protest from the drivetrain. So I stopped and tried again to put the tranny in Drive but again this awful sound from the transfer case and drivetrain.

    I place the tranny into Neutral and switch the Selec-Trac into 4HI it made a loud bang and move the vehicle forward with no problem. I drove about a mile or so and tried it all over again 4HI to 2WD and it worked! I drove another couple miles and when I stop again at a intersection it banged back out of 2WD with the chain noise..etc..

    What the hell is going on?

    Vacuum motor disengaging from 2WD??
    I was not going fast when I enter the underpass, I have BFG 33x12.5 15 and this snow slush water could not have came up 3/4 of the tire height.
    I did have to drive say 50 yards thru it. It is raining here and we got snow fall melting fast enough to flood for a mintue

    What do you all think?

    Is this Vacuum operated Selec-Trac - really - Selec-Trac II ......fulltime 4wd drive, 4wdLO, 2wdHI ??

    Does anyone know where vacuum motors replacements can be found ???
    Last edited by BossVP; 01-15-2014, 02:23 PM.

  • #2
    Correct me if I am wrong... But it sounds like you have the command trac system. Which is a full time 4wd case with a vac switch under the dash to engage the axle. And a lever on the floor that says hi or low. If that is the case you have... There's a vac motor that keeps the rear wheels engaged... When that craps out well... You'll experience the bangs and loss of power to the rear wheels. Selec trac is the np208 part time case which has the lever on the floor to select 2hi, 4 hi, n, or 4 lo. That is what I did with mine. The 208 swap is extremely easy.